I had a similar problem with Yoast, but I guess a bit simpler since my fix was easier.
The error that I got in Webmaster Tools referenced the Post_Tag.xml. I realized that the site I was looking at wasn’t even using “tags” on their blog, so I went into Yoast SEO settings and just checked off the Tags (post_tag) under Exclude Taxonomies and saved changes. At Webmaster Tools I resubmitted my sitemap and it worked perfectly fine.
I didn’t need to do anything other than what’s noted above, and I’m not a dev, so maybe this is an easy fix for people with a similar problem who are not so technical?
